The present invention relates to a gas-detecting element and a gas-detecting device for measuring the concentration of a detection object gas in a gas atmosphere, particularly to a gas-detecting element and a gas-detecting device suitable for directly measuring the concentration of nitrogen oxides in a combustion exhaust gas emitted from automobiles, etc.
So-called gas sensors with high gas selectivity capable of electrochemically detecting a particular gas by using solid electrolyte substrates have recently been proposed actively. Particularly, gas sensors capable of measuring the concentration of total NOx in an exhaust gas from automobiles without affected by other gases are strongly demanded.
Thus, the inventors previously proposed a mixed-potential-type NOx sensor comprising an oxygen-ion-conductive zirconia solid electrolyte operable at high temperatures in JP 9-274011 A. This NOx sensor has a basic structure, which comprises a NOx-sensing electrode and a reference electrode formed on an opposite or same surface of a zirconia solid electrolyte substrate as the NOx-sensing electrode. In this NOx sensor, a sensing electrode is, of course, exposed to a detection gas (gas to be detected), and a reference electrode can be simultaneously exposed to a detection gas, if the reference electrode is active with only oxygen. Because the NOx-sensing electrode is active with NOx and oxygen, and because the reference electrode is active only with oxygen, output (potential difference) can be obtained due to the difference in chemical potential between both electrodes. Accordingly, the measurement of potential difference between both electrodes leads to the detection of the NOx concentration in the detection gas. Incidentally, when the reference electrode is also active with NOx, the same NOx sensitivity can be obtained if isolated from the detection gas.
At the time of detecting a gas by the sensing electrode of the above mixed-potential-type NOx sensor, however, NO is subjected to reactions represented by the following formulae (1) and (2):
O2+4exe2x88x92xe2x86x922O2xe2x88x92xe2x80x83xe2x80x83(1), and
2NO+2O2xe2x88x92xe2x86x922NO2+4exe2x88x92xe2x80x83xe2x80x83(2),
and NO2 is subjected to reactions represented by the following formulae (3) and (4):
2O2xe2x88x92xe2x86x92O2+4exe2x88x92xe2x80x83xe2x80x83(3), and
2NO2+4exe2x88x92xe2x86x922NO+2O2xe2x88x92xe2x80x83xe2x80x83(4).
As a result, sensor outputs with NO and NO2 at the time of detecting a gas are just opposite in polarity. When the concentration of total NOx is detected in an exhaust gas emitted from vehicles, the coexistence of NO and NO2 causes interference if no measure is taken, failing to detect the concentration of total NOx precisely.
Accordingly, JP 9-274011 A proposes a laminate-type gas-detecting device. According to the principle of this laminate-type gas-detecting device, oxygen from air is introduced into a gas detection chamber using an electrochemical oxygen pump. As a result, reducing gases such as HC (hydrocarbons), CO (carbon monoxide), etc. in the detection gas are oxidized to be harmless. Simultaneously, NO in NOx is electrochemically converted to NO2, so that NOx becomes consisting only of NO2. After this treatment for turning a detection gas to contain only one detection object gas, the NO2 concentration is measured from the potential difference between the NOx-sensing electrode and the reference electrode, thereby determining the concentration of total NOx.
In such NOx-detecting element or such laminate-type NOx gas-detecting device, its detection performance, namely sensitivity and its stability and response, is particularly governed by the performance of a sensing electrode. Conventionally reported as the sensing electrodes of such mixed-potential-type NOx sensors are, for instance, NiCr2O4 (SAE Paper No. 961130), Ptxe2x80x94Rh alloys or cermet electrodes comprising Ptxe2x80x94Rh alloys to which a zirconia solid electrolyte is added (JP 11-72476 A). These sensing electrodes have excellent sensitivity. However, further improvement is needed with respect to the stability of sensitivity of sensing electrodes. For this purpose, it is important to improve the stability of an electrode material per se, and the bonding stability of interface (electrode interface) between a solid electrolyte substrate and a sensing electrode. Particularly when metal oxides are used for the electrodes, it has conventionally been difficult to control the bonding stability of this electrode interface. This is because there is generally weak bonding between metal oxides and solid electrolyte substrates, resulting in the likelihood that peeling and cracking occur in their interface.
To improve the response of gas detection, it is necessary to reduce the interface impedance of the electrode in the gas sensor. For this purpose, increase in an electrode area and the elevation of operation temperatures have been investigated. However, in a mixed-potential-type sensor, the higher the temperature, the lower the gas sensitivity. In addition, to increase an electrode area, it is necessary to make a sensor element larger. The increase of the sensor element deteriorates the uniformity of the temperature distribution of the sensor element, resulting in the variation of performance and instability.
As described above, though there are materials excellent in sensitivity for the mixed-potential-type NOx sensor, further improvement is needed with respect to the stability of sensitivity. Particularly when a metal oxide electrode is used as a sensing electrode, there is poor bonding stability with the solid electrolyte substrate, resulting in the variation of detection performance and decrease in yield. Therefore, it is desired not only to improve interface stability between the sensing electrode and the solid electrolyte substrate, but also to reduce the variation of characteristics that are caused during a production process for some reasons. Further, it is desired to improve gas response without making the sensor element larger, and without accompanying decrease in gas sensitivity.
Though the importance of stability and response of the sensing electrode has been described above, such characteristics are not required only to the sensing electrode. In the case of the NOx sensor, for instance, it is important to improve the stability and response of a reference electrode serving as a reference for electrode potential, and an oxygen-sensing electrode for making compensation for oxygen concentration, etc., because these characteristics also affect the performance of the NOx sensor.
In the case of a NOx sensor mounted onto a vehicle, oxygen concentration in the detection gas widely varies, and thus the influence of the concentration of coexisting oxygen cannot be neglected. In the NOx sensor of this type, a reference electrode active only with oxygen is disposed in a portion close to the NOx-sensing electrode, and the measurement of potential between the NOx-sensing electrode and the reference electrode leads to the determination of NOx concentration in the detection gas. By mounting an oxygen-sensing electrode active with oxygen but inactive with NOx near the NOx-sensing electrode within a detection chamber, by measuring potential difference E2 between the reference electrode and the oxygen-sensing electrode disposed in an air duct, and potential difference E1 between the reference electrode and the NOx-sensing electrode, and by arithmetically treating these difference (E1xe2x88x92E2), it is possible to compensate the variation of oxygen concentration. The measurement using such electrode inactive with a detection gas enables high-precision measurement of the concentration of a detection object gas even with a detection gas such as an exhaust gas from automobiles, etc. in which oxygen concentration varies.
However, if the reference electrode or the oxygen-sensing electrode becomes considerable active with NOx (exhibits mixed potential), its influence decreases the precision of NOx detection. To improve the sensitivity of the NOx sensor, it is desirable to reduce the activity of the reference electrode or the oxygen-sensing electrode with NOx. The activity of the reference electrode or the oxygen-sensing electrode with NOx is presumed to be generated by the contamination of these electrodes, vapor deposition from other electrodes evaporated during the sintering step, etc. Though the contamination can be prevented by control of the production process, the vapor deposition from other electrodes during the sintering step is inevitable because of the restrictions of the sintering conditions of substrates, sensing electrode characteristics, etc. Accordingly, it is important to minimize the activity of the reference electrode and the oxygen-sensing electrode with a detection gas such as NOx, etc., during the production and operation.
In the laminate-type NOx gas-detecting device, its detection performance is affected by the performance of the conversion electrode for electrochemically converting NO to NO2 or NO2 to NO in the NOx. Therefore, the conversion electrode should carry out the desired oxygen pumping. The factors for varying oxygen pumping are the change of electric resistance of a conversion electrode per se, the change of interface resistance between a conversion electrode and a solid electrolyte substrate, the change of bulk resistance of a solid electrolyte per se, etc. Also, the conversion electrode should have not only an excellent oxygen pumping function, but also excellent performance of adsorption and desorption of NO for electrochemically converting NO in the NOx to NO2. From these facts, the conversion electrode should be excellent in electrochemical stability characteristics.
However, it is necessary to further improve the conversion electrode with respect to stability of sensitivity. For this purpose, it is important to have good bonding stability of interface (electrode interface) between the solid electrolyte substrate and the conversion electrode. By the difference in a sintering shrinkage ratio and a thermal expansion coefficient between the solid electrolyte substrate and the conversion electrode, it is difficult to maintain the bonding stability of electrode interface for a long period of time. Particularly when the conversion electrode contains a metal oxide, there is weak bonding with the solid electrolyte substrate, making it likely to cause peeling and cracking in the interface.
Further, when the conversion electrode comes into direct contact with a strong reducing gas such as HC (hydrocarbons), CO (carbon monoxide), etc., its performance of adsorption and desorption of NO is likely to be remarkably changed. The laminate-type, gas-detecting device of JP 9-274011 A oxidizes a reducing gas such as HC (hydrocarbons), CO (carbon monoxide), etc. in the detection gas to be harmless. However, when this gas-detecting device is used for automobiles, the temperature of the gas-detecting device is not so elevated at the time of starting an engine that the conversion pump element does not fully work. Thus, when exposed to HC and CO, the adsorption and desorption of NO is remarkably changed.
Accordingly, an object of the present invention is to provide a gas-detecting element and a gas-detecting device excellent in the bonding stability of interface between an electrode and a solid electrolyte substrate, with the activity of a reference electrode or an oxygen-sensing electrode with a detection object gas suppressed, thereby exhibiting stable sensitivity and excellent response performance.
The first gas-detecting element of the present invention comprises an oxygen-ion-conductive solid electrolyte substrate, a sensing electrode fixed onto the solid electrolyte substrate and active with a detection object gas and oxygen, and a reference electrode fixed onto the solid electrolyte substrate and active with at least oxygen, to determine the concentration of the detection object gas from the potential difference between the sensing electrode and the reference electrode, wherein the sensing electrode and/or the reference electrode being covered by an electrode-coating layer made of an oxygen-ion-conductive solid electrolyte, the electrode-coating layer having a portion bonded to the solid electrolyte substrate directly or via an electrode underlayer made of an oxygen-ion-conductive solid electrolyte.
By covering a sensing electrode with an electrode-coating layer made of an oxygen-ion-conductive solid electrolyte, it is possible to reduce the bonding instability of interface between a solid electrolyte substrate and the sensing electrode, which is caused by thermal stress due to the difference in a thermal expansion coefficient between them. Though there is an electrode interface (three-phase interface) only in a bonding interface between a sensing electrode and a solid electrolyte substrate in a conventional gas-detecting element, a bonding interface between a sensing electrode and an electrode-coating layer also serves as an electrode interface in the gas-detecting element of the present invention, resulting in drastic increase in an electrode interface area. Accordingly, the electrode impedance can be reduced, resulting in improvement in gas response.
By covering a reference electrode with an electrode-coating layer made of an oxygen-ion-conductive solid electrolyte, the stability of interface and the gas response are also improved, like the sensing electrode. When the reference electrode is also exposed to a detection gas (a gas to be detected), the decrease of interface impedance leads to the increase of reaction sites of oxygen, as long as oxygen concentration is sufficiently higher than the concentration of a detection object gas in the detection gas. In this case, because reaction sites of a low concentration of a detection object gas (for instance, NOx) are not substantially influenced, the activity of the reference electrode to the detection object gas decreases. Further, the electrode-coating layer prevents contamination to the reference electrode during production processes and use of a sensor, so that the sensitivity of the reference electrode to the detection object gas can be kept low, resulting in improvement in the precision and stability of a sensor.
The second gas-detecting element of the present invention comprises an oxygen-ion-conductive solid electrolyte substrate, a sensing electrode fixed onto the solid electrolyte substrate and active with a detection object gas and oxygen, an oxygen-sensing electrode fixed onto the solid electrolyte substrate and active with at least oxygen, a reference electrode positioned in an atmosphere separated from a detection object atmosphere and active with oxygen, to determine the concentration of the detection object gas from the difference (E1xe2x88x92E2) between a potential difference E1 between the sensing electrode and the reference electrode and a potential difference E2 between the oxygen-sensing electrode and the reference electrode, wherein the sensing electrode and/or the oxygen-sensing electrode being covered by an electrode-coating layer made of an oxygen-ion-conductive solid electrolyte, the electrode-coating layer having a portion bonded to the solid electrolyte substrate directly or via an electrode underlayer made of an oxygen-ion-conductive solid electrolyte.
By covering a sensing electrode and/or an oxygen-sensing electrode with an electrode-coating layer, the bonding stability of interface between a sensing electrode and/or an oxygen-sensing electrode and a solid electrolyte substrate, and gas response are improved. Also, the oxygen-sensing electrode exposed to a detection gas has lowered activity with the detection object gas, and it is possible to prevent the oxygen-sensing electrode from having activity with a detection object gas by contamination, thereby improving the precision and stability of the detection element.
Preferred examples of the gas-detecting element of the present invention are as follows:
(1) The electrode-coating layer covering the sensing electrode and/or the reference electrode is in the form in which a detection gas can reach a three-phase interface between each electrode and the solid electrolyte substrate, the electrode underlayer or the electrode-coating layer.
(2) At least one of the sensing electrode, the reference electrode and the oxygen-sensing electrode is fixed onto the solid electrolyte substrate via an electric insulating layer.
(3) At least one of the sensing electrode, the reference electrode and the oxygen-sensing electrode is fixed in a recess formed on the solid electrolyte substrate.
(4) The electrode-coating layer covering the sensing electrode has a porosity of 10-50%.
(5) The electrode-coating layer covering the sensing electrode has an average thickness of 3-20 xcexcm.
(6) The electrode-coating layer covering the reference electrode or the oxygen-sensing electrode has a porosity of 0-50%.
(7) The electrode-coating layer covering the reference electrode or the oxygen-sensing electrode has an average thickness of 1-20 xcexcm.
(8) The electrode-coating layer covering at least one of the sensing electrode, the reference electrode and the oxygen-sensing electrode has an average thickness of 5-100 xcexcm, and the electrode-coating layer is provided with gas-diffusing pores, a ratio (Sh/Se) of the total opening area (Sh) of the gas-diffusing pores to the area (Se) of the sensing electrode being 0.05-0.28.
(9) An upper surface of at least one electrode of the reference electrode and the oxygen-sensing electrode exposed to a detection gas is covered by a dense electrode-coating layer, and part of side surfaces of the electrode is exposed.
(10) A plurality of sensing electrodes are formed via the electrode-coating layer covering the sensing electrode.
(11) The electrode-coating layer covering at least one of the sensing electrode, the reference electrode and the oxygen-sensing electrode is made of a zirconia solid electrolyte containing at least one selected from the group consisting of yttria, ceria, magnesia and scandia as a stabilizer.
(12) The electrode-coating layer covering the sensing electrode contains a precious metal active with the detection object gas and oxygen.
(13) The electrode-coating layer covering at least one of the sensing electrode, the reference electrode and the oxygen-sensing electrode contains a precious metal inactive with the detection object gas but active with oxygen.
(14) The electrode underlayer is made of a zirconia solid electrolyte containing at least one selected from the group consisting of yttria, ceria, magnesia and scandia as a stabilizer.
(15) The sensing electrode is made of a metal oxide and/or a precious metal active with a detection object gas and oxygen.
(16) The detection object gas is any of nitrogen oxides, hydrocarbon, carbon monoxide or ammonia.
The first gas-detecting device of the present invention comprises (a) a gas-measuring chamber defined by first and second oxygen-ion-conductive solid electrolyte substrates disposed with a predetermined gap; (b) a gas inlet so that a detection gas flows into the gas-measuring chamber with a predetermined gas diffusion resistance; (c) a gas-detecting element comprising a sensing electrode fixed onto the first solid electrolyte substrate such that it is exposed to an atmosphere in the gas-measuring chamber, and active with a detection object gas and oxygen, and a reference electrode fixed onto the first solid electrolyte substrate and active with at least oxygen; (d) a detection-object-gas-converting pump element comprising (i) a detection-object-gas-converting electrode fixed onto the second solid electrolyte substrate such that it is exposed to an atmosphere in the gas-measuring chamber, and active with a detection object gas and oxygen, and (ii) a detection-object-gas-converting counter electrode fixed onto the second solid electrolyte substrate such that it is exposed to an atmosphere containing oxygen and/or an oxide gas, and active with oxygen, which can select the oxidation or reduction of a detection object gas depending on conditions; (e) a means for measuring the potential difference between the sensing electrode and the reference electrode; and (f) a voltage-applying means for driving the conversion pump element, to detect the potential difference between the sensing electrode and the reference electrode while applying predetermined voltage to the conversion pump element, thereby determining the concentration of the detection object gas in the detection gas, wherein the sensing electrode being covered by an electrode-coating layer made of an oxygen-ion-conductive solid electrolyte, and the electrode-coating layer having a portion bonded to the first solid electrolyte substrate directly or via an electrode underlayer made of an oxygen-ion-conductive solid electrolyte.
The detection object gas suitable for the above gas-detecting device is NOx.
The second gas-detecting device of the present invention comprises (a) a gas-measuring chamber defined by first and second oxygen-ion-conductive solid electrolyte substrates disposed with a predetermined gap; (b) a gas inlet provided in the gas-measuring chamber such that a detection gas flows into the gas-measuring chamber with a predetermined gas diffusion resistance; (c) a gas-detecting element comprising a sensing electrode fixed onto the first solid electrolyte substrate such that it is exposed to an atmosphere in the gas-measuring chamber, and active with a detection object gas and oxygen, and a reference electrode fixed onto the first solid electrolyte substrate and active with at least oxygen; and (d) a detection-object-gas-converting pump element comprising (i) a detection-object-gas-converting electrode fixed onto the second solid electrolyte substrate such that it is exposed to an atmosphere in the gas-measuring chamber, and active with a detection object gas and oxygen, (ii) a detection-object-gas-converting counter electrode fixed onto the second solid electrolyte substrate such that it is exposed to an atmosphere containing oxygen and/or an oxide gas, and active with oxygen, which can select the oxidation or reduction of a detection object gas depending on conditions; (e) a means for measuring the potential difference between the sensing electrode and the reference electrode; and (f) a voltage-applying means for driving the conversion pump element, thereby detecting the potential difference between the sensing electrode and the reference electrode while applying predetermined voltage to the conversion pump element, to determine the concentration of the detection object gas in the detection gas; the electrode for converting the detection object gas being covered by an electrode-coating layer made of an oxygen-ion-conductive solid electrolyte, through which the detection object gas can reach to the electrode; and the electrode-coating layer having a portion bonded to the second solid electrolyte substrate directly or via an electrode underlayer made of a solid electrolyte.
Preferred examples of the gas-detecting device of the present invention are as follows:
(1) The electrode-coating layer covering the detection-object-gas-converting electrode is preferably in such a form that a detection gas can reach a three-phase interface of the solid electrolyte substrate, the electrode underlayer or the electrode-coating layer and each electrode.
(2) The electrode-coating layer is constituted by a porous solid electrolyte film layer having pores through which the detection object gas can be diffused, the porous solid electrolyte film layer having a porosity of 10-50% and an average thickness of 3-20 xcexcm.
(3) The electrode-coating layer covering the detection-object-gas-converting electrode is made of a zirconia solid electrolyte containing as a stabilizer at least one selected from the group consisting of yttria, ceria, magnesia and scandia.
(4) The electrode-coating layer covering the detection-object-gas-converting electrode comprises (a) at least one precious metal selected from the group consisting of platinum, rhodium, iridium, gold and alloys containing these metals, and/or (b) at least one metal oxide selected from the group consisting of Cr2O3, NiO, NiCr2O4, MgCr2O4 and FeCr2O4 in a range of 1-50% by volume based on 100% by volume of the solid electrolyte.
(5) The electrode underlayer is made of a zirconia solid electrolyte containing as a stabilizer at least one selected from the group consisting of yttria, ceria, magnesia and scandia.
(6) Said electrode underlayer comprises (a) at least one precious metal selected from the group consisting of platinum, rhodium, iridium, gold and alloys containing these metals, and/or (b) at least one metal oxide selected from the group consisting of Cr2O3, NiO, NiCr2O4, MgCr2O4 and FeCr2O4 in a range of 0.1-20% by volume based on 100% by volume of the solid electrolyte.
(7) The detection-object-gas-converting electrode is made of at least one precious metal selected from the group consisting of platinum, rhodium, iridium, gold and alloys containing these metals.
(8) The detection-object-gas-converting electrode and a layer for coating this electrode are made of a zirconia solid electrolyte containing the same stabilizer, the stabilizer being at least one selected from the group consisting of yttria, ceria, magnesia and scandia.
(9) The gas-detecting device further comprises a means for heating at least the gas-detecting element and the detection-object-gas-converting pump element to a predetermined temperature.
(10) The detection object gas is any of a nitrogen oxide gas, a hydrocarbon gas, a carbon monoxide gas and an ammonia gas.
(11) The detection object gas is nitrogen oxide, and the oxidation reaction of NO to NO2 or the reduction reaction of NO2 to NO in the detection gas by the conversion pump element can be selected depending on conditions.
(12) The above gas-detecting element is the gas-detecting element of the present invention.
(13) When the concentration of a reducing detection object gas is measured by the gas-detecting device of the present invention, at least a sensing electrode is exposed to an atmosphere containing 0.1% by volume or more of oxygen, to measure potential difference between the sensing electrode and the reference electrode.
The electrode-coating layer prevents the direct contact of the conversion electrode with a detection gas. By driving a conversion pump element and/or a gas-treating pump element, oxygen is pumped into the gas-measuring chamber, in which a reducing gas in the detection gas can be oxidized. Particularly in the case of driving with a conversion electrode of a conversion pump element as an anode, the formation of the electrode-coating layer increases oxidation efficiently of a reducing gas by oxygen pumped from the conversion electrode. Accordingly, a detection gas containing a high concentration of a reducing gas is not directly contacted with the conversion electrode, thereby suppressing the remarkable change of adsorption and desorption performance of NO.
Because bonding interface between the conversion electrode and the electrode-coating layer serves as electrode interface, an electrode interface area can be drastically increased, thereby suppressing the variation of interface impedance between the conversion electrode and the solid electrolyte substrate.